1. Field of the Invention
This invention relates generally to beam steering controllers for phased array radar systems, and more particularly relates to a method for reducing phase error of digitally controlled phase shifters in beam steering controllers for phased array radar.
2. Prior Art
Conventional radar systems have typically involved the use of a constantly rotating mechanically steered radar dish, which can gather information over broad areas about a large number of objects. However, the update rate, the rate at which a radar takes new readings of targets, is limited by the rate at which the radar dish turns on its shaft. A single mechanically steered radar can provide limited information concerning one or a few closely spaced objects, but in a number of circumstances, there is a necessity for tracking a large number of targets over broad areas. Until recently, only groups of radars, each assigned to one or several of the targets could serve that purpose. Innovations in phased array radar have improved the information gathering ability so that hundreds of targets scattered through a broad volume of space can be watched simultaneously, with the radar beam being electronically redirected from target to target in a matter of microseconds.
The electronic beam steering of phased array radar takes advantage of the principle that wave patterns resulting from adjacent radiating sources will interfere. Superposition of the wave patterns determines how they will interact. If the individual wave forms are in phase, so that crests coincide with crests, and troughs coincide with troughs, the patterns will result in constructive interference, but if the wave forms are out of phase, destructive interference will result, with the signals yielding a weaker signal or cancelling each other entirely.
If the signals from a phased array of radiating elements leave the array in phase, they add up in phase along the boresight of the array. Delaying of signals from each of the radiating elements by amounts that increase steadily across the face of the array causes a signal to lag a fraction of a wavelength behind the signal from an adjacent element, changing the relative phases of the signals. The direction of the radar signal will then not be straight down the boresight of the antenna, but off to the side in the direction of the increasing phase delay. The phase slope is the rate of change of the phase angle across the face of the antenna.
This type of phase lag steering is implemented by a phase shifter, which conventionally consists of variable susceptance elements which can be selectively introduced in the path of the signal as it travels on its way from an oscillator or amplifier to an individual radiating element. Thus, increasing the group delay of the wave guide or cable through which the signal which travels introduces a delay or phase shift in the transmission of the signal. The phase shifting can thus proceed in steps, using a hierarchy of susceptances attached to each element. The switching of the individually selected susceptance elements can be digitally controlled, by a central computer.
A typical phase shifting radar uses three bit phase shifters, phase shifters with 2.sup.3 equivalent path lengths. Although the switching is initially determined digitally, and even though the implementation of susceptance selection can be performed mechanically or electronically, e.g. by electromechanical or diode switches, the ultimate control of phase shifting is essentially analog, requiring a large quantity of microwave circuit elements for an entire array. Such a radar system is described in Brookner, "Phased Array Radars" Scientific American, Vol. 252, No. 2, Feb. 1985, pp. 94-102. As thousands of individual elements can be included in such an array, with each individual element being controlled by switches, each having microwave circuit elements to be switched in to determine varying signal delays, it would be desirable to provide acceptable antenna array performance in the presence of failures in the phase shifters. Even if each phase shifter element is very reliable, the need for long periods of maintenance free operation, typically years, and the large number of elements makes the probability of a few failures very high. Thus to extend the time between maintenance actions, the phased array could be implemented to sense failures in the phase shifters and correct a small number of failures, typically one percent of the total phase or elements.
One recent method of digitally controlling the phase of individual radiating elements in a phased array is by way of introduction of a cascaded sequence of amounts of binarily weighted group delays at each radiating element. As each quantity of binary weighted group delay or phase shift or either switched in or out of position according to commands by a central radar data processor, the phase control of each individual element in the array is subject to direct digital control from the central processor. A common failure mechanism which arises in the implementation of such a design is that the phase shifting element may become stuck in the wrong phase position, causing an error in the phase angle of the individual radiating element. It would therefore be desirable to monitor the failure status of each phase changer of each phase shifter, and reduce or eliminate any phase error due to such phase changer failures. The method and system of the invention fills this need by providing alternative phase shift commands to reduce phase error resulting from such phase shifter failures.